As a display mode for a liquid crystal display device (LCD), various modes have been proposed such as twisted nematic (TN) mode, supertwisted nematic (STN) mode, active matrix (AM-LCD) mode, etc., which have been put to practical use. Of these practical modes, the STN mode having the orientation of liquid crystal molecules in the upper and lower substrates twisted at 180-270.degree. which was proposed by T. J. Scheffer, et al. (Appl. Phys. Lett. 45 (10), 1021 (1984)) has been used for the LCD of personal computers or the like.
In recent years, the STN mode is prevailing in the use of a liquid crystal display device for hand-held terminals, e.g. electronic notepads, laptop computers which are often used in the open air.
The liquid crystal compositions used in the STN mode have required the following general characteristics (1) to (4). In addition, the liquid crystal compositions used in the STN mode in the open air have required the following general characteristics (5) and (6).
(1) A steepness of the liquid crystal composition should be improved as well as possible in order to increase a contrast of the liquid crystal display device. (The voltage-transmittance characteristics (steepness) should be steep.)
(2) A viscosity of the liquid crystal composition should be reduced as low as possible in order to decrease a response time of the liquid crystal display device.
(3) An optical anisotropy of the liquid crystal composition can take a suitable value depending on a cell thickness of the liquid crystal display device in order to optimize a contrast of the liquid crystal display device.
(4) The liquid crystal composition should exhibit a nematic phase in a wide temperature range in order to extend an environmental temperature at which the liquid crystal display device is used. (The upper-limit temperature of the nematic phase is increased and the lower-limit temperature of the nematic phase is reduced.)
(5) A threshold voltage of the liquid crystal composition should be reduced in order to provide a smaller-sized battery serving as a power for driving the liquid crystal display device. Further, a temperature dependence of the threshold voltage of the liquid crystal composition should be reduced.
(6) The liquid crystal composition should have a high stability to heat and ultraviolet ray in order to prevent degradation of the liquid crystal composition and maintain semi-permanently the display quality of the liquid crystal display device.
The liquid crystal compositions for STN having relatively low threshold voltage and relatively good temperature characteristics of the threshold voltage are disclosed in Japanese Patent Kokai 7-300582, Japanese Patent Kokai 7-300585 and Japanese Patent Kokai 7-300584. Those compositions have the problem of poor stability to heat and ultraviolet ray, as shown in comparative examples which will be given later.
As liquid crystal compounds for low-voltage driving in various modes including active matrix and STN modes, WO 96/11897 discloses new liquid crystalline compounds having high dielectric anisotropy and very low viscosity as well as liquid crystal compositions containing said compounds. Composition Examples 19-22 of WO 96/11897 illustrate the compositions which contain the compounds having --CF.sub.2 O-- as a linking group and having 3,5-difluoro-4-cyanophenyl at the end group. The composition shown in Composition Example 19 is analogous to that of the present invention, but has the problems of poor steepness, high threshold voltage and high temperature dependence of the threshold voltage.
Thus there is a continuing need for an improved liquid crystal composition.